Does being fat make you more jolly?

The news has been full of headlines this week about a gene that supposedly explains the "fat and jolly" stereotype. Can it be true? We seek out the science behind the story.

What was the actual discovery behind these stories?
David Meyre of McMaster University in Hamilton, Canada, and his colleagues found that a gene mutation associated with obesity is also associated with an 8 per cent decrease in the risk of depression. "This is the first gene ever convincingly associated with depression," says Meyre. Early candidates for "depression genes" have failed to stand up, he says ? which has been frustrating, as family studies show that genetics determines nearly half our risk of depression.

How did they make a genetic connection between obesity and depression this time round?
The FTO gene makes a protein associated with obesity and fat mass. In 2007, Meyre was part of a team that discovered mutations in FTO that, he says, are "clearly associated with an increased risk of obesity".

Obesity is a risk factor for depression, and vice versa. What's more, FTO is heavily expressed in the brain. Meyre put these facts together and came up with the hypothesis that the variant gene would increase the risk of depression.

Instead, it decreased it. The team was so surprised when this emerged from two large Canadian studies that they looked at two more, by British and Swiss researchers, in the end pooling data from 28,000 people worldwide. All four studies gave the same result: having one copy of this mutant in your genome decreases the risk of depression by 8 per cent; two copies doubles that dip. The effect was independent of whether a person was actually obese or not.

What's the explanation?
Meyre suspects that the gene, which methylates DNA and therefore turns other genes off and on, is able to independently influence several disorders ? all of which may involve "appetitive" drives such as eating, and the mood that goes with them.

How does the FTO gene affect humanity as a whole?
Meyre is happy to call it a happiness gene. "It makes a modest contribution [to happiness]," he says. Based on its prevalence among ethnic groups, it should prevent 6.7 per cent of the cases of depression that would otherwise afflict Africans, 5.3 per cent of cases in Europeans, and 2.2 per cent in Chinese.

This varying prevalence suggests it was selected for in the past by different histories, probably involving famine, when being better at storing fat was an advantage. A tendency to have a positive attitude as well, Mayre speculates, also had survival value.

Is this really why fat people have a reputation for being more jolly?
No, for the simple reason that, in Western society, they are not: obese people are more likely to be depressed. Also, not being depressed isn't quite the same as being jolly or happy. And this study certainly does not show that getting fatter makes you happier, only that a gene that predisposes you to obesity also predisposes you to be less likely to be depressed.

Still, in the distant past, chronically underfed peasants may have noticed that fat people tended to be happier, and so the legend of the jolly, fat person ? Santa Claus and the like ? was born. Of course, anyone who got fat then probably had other reasons to be happy.

Journal reference: Molecular Psychiatry, doi.org/jtn

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